Earnings Per Share - Summary of Basic and Diluted EPS (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net Income	$ 56.0		$ 161.0	$ 115.0
Less preferred stock dividend	7.0		21.0	0.0
Net Income (Loss) from Continuing Operations Available to Common Shareholders, Basic	$ 49.0		$ 140.0	$ 115.0
Weighted-average common shares outstanding - basic (in shares)	214,370,000		214,370,000	58,333,000
Dilutive effect of unvested restricted stock and unvested performance restricted stock (in shares)	49,000		21,000	37,000
Dilutive effect of stock options (in shares)	0		0	68,000
Weighted-average shares outstanding - diluted (in shares)	214,418,693		214,390,931	58,438,000
Net income per common share:
Basic (in USD per share)	$ 0.23		$ 0.65
Diluted (in USD per share)	$ 0.23		$ 0.65
Common Stock Warrants Outstanding	70,000,000		70,000,000
Preferred Stock
Net income per common share:
Antidilutive securities excluded from computation of EPS, less than, amount (shares)			391,000
Stock Option Awards
Net income per common share:
Antidilutive securities excluded from computation of EPS, less than, amount (shares)	1,384,000		609,000
Predecessor
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net Income		$ 61.0		$ 115.0
Less preferred stock dividend		0.0		$ 0.0
Net Income (Loss) from Continuing Operations Available to Common Shareholders, Basic		$ 61.0
Weighted-average common shares outstanding - basic (in shares)		58,335,216		58,332,666
Dilutive effect of unvested restricted stock and unvested performance restricted stock (in shares)		52,000
Dilutive effect of stock options (in shares)		92,000
Weighted-average shares outstanding - diluted (in shares)		58,478,612		58,437,832
Net income per common share:
Basic (in USD per share)		$ 1.06		$ 1.98
Diluted (in USD per share)		$ 1.06		$ 1.98
Predecessor | Stock Option Awards
Net income per common share:
Antidilutive securities excluded from computation of EPS, less than, amount (shares)		0		0